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BDS2 RPD > RPD - Retention > Flashcards

RPD - Retention Flashcards

1
Q

What is meant by retention?

A
  • A term used to describe the resistance of a denture to lifting away from the tissues (vertical dislodging forces)
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2
Q

In what 3 ways can retention be achieved?

A
  • Mechanical means
  • Muscular means
  • Physical means
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3
Q

How can retention be achieved mechanically?

A
  • Clasps
  • Clasps will eventually fatigue, oral muscles will kick in and help here - as long as the denture is made to accommodate this
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4
Q

How can retention be achieved using muscular forces?

A
  • Shape of the denture

- This comes through time

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5
Q

How can retention be achieved through physical forces?

A
  • Coverage of the mucosa
  • Using external forces of adhesion, cohesion, surface tension and atmospheric pressure on the impression surface of the denture.
  • Adhesion - surface forces of saliva on denture and mucosa
  • Cohesion - surface forces of saliva on denture and mucosa
  • Atmospheric pressure - resistance to displacing forces
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6
Q

What is meant by cohesion?

A
  • The sticking together of particles of the same substance
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7
Q

What are the 2 classifications of retention?

A
  • Direct

- Indirect

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8
Q

What is DIRECT retention?

A
  • Resistance to vertical displacement of the denture (clasps, physical forces - oral musculature)
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9
Q

What is INDIRECT retention?

A
  • Resistance to ROTATIONAL displacement of the denture

- Things that prevent the denture from rotating out during function

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10
Q

What are guide planes (direct retention)?

A
  • Two surface on a saddle parallel to each other
  • They tend to occur naturally
  • As long as you maintain the space, you keep the frictional contact and so make retention longer
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11
Q

Where would you find direct retention?

A
  • Close to the base
  • Adjacent to the abutment teeth
  • Chances of it being interproximal are unlikely as dental materials tend to be rigid
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12
Q

What is the principal method for providing mechanical retention?

A
  • By using clasps
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13
Q

What is a clasp?

A
  • A metal arm that, when in position contacts the tooth preventing removal of the denture base
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14
Q

To be effective as a retainer, where is a clasp placed on a tooth?

A
  • Below the bulbous part of a tooth (the undercut)
  • The bulbous portion of the tooth required identification
  • To accomplish this a SURVEY of the cast is performed
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15
Q

What 2 ways can clasps be included in a denture?

A
  • Can be made in wrought metal and incorporated into the denture base
  • OR can be included as part of a cast denture base
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16
Q

Where do clasps approach the undercuts from?

A
  • Either the occlusal surface (occlusally approaching) or the gingival margin (gingivally approaching)
17
Q

Clasps work most efficiently when in conjunction with what?

A
  • When in conjunction with a rest (supporting component)
18
Q

What is meant by reciprocation in relation to RPD’s.

A
  • As part of the clasp assembly each clasp unit will have an opposing element to the retentive clasp arm to prevent pressure from the clasp acting on the tooth
19
Q

What is the pattern of retention? (4 points)

A
  • Ideal number of retainers = 3
  • Always have one that is going to prevent dislodging
  • Want it in the shape of a triangle and want to make the triangle as large as possible
  • However, not always possible to create a triangle e.g. in a bilateral free-end saddle
20
Q

When is the stress relieving clasp system (RPI) used?

A
  • Used in free-end saddle designs to prevent stress on the last abutment tooth
21
Q

What components make up the stress relieving clasp system (RPI)? (3 points)

A

Rest (occlusal)

  • Mesial of the tooth
  • Rounded on the impression surface

Proximal Plate (adjacent to saddle)

  • Guide surface of 2-3mm
  • Undercut to permit movement

I-bar clasp (gingivally approaching)

  • Greatest prominence of the tooth contour
22
Q

What is the RPI stress relieving clasp system used for?

A
  • Used in free-end saddle designs to prevent stress on the last abutment tooth
23
Q

What is indirect retention obtained by?

A
  • Obtained by the extension of a partial denture base to provide the fulcrum of a class II lever. (the retainer(s) providing direct retention lie between the fulcrum and that part of the denture which is subject to the displacing force)
24
Q

What are indirect retainers principally known as?

A
  • Supporting components
25
Q

What are examples of indirect retainers? (5 points)

A
  • Major connectors
  • Minor connectors
  • Rest
  • Saddle
  • Denture base
26
Q

Is retention required in every adjacent (abutment) tooth to the denture saddle?

A
  • No
27
Q

What is the minimum amount of clasps and their position to be sufficient depending on the saddle length?

A
  • One clasp on each side of the arch
28
Q

How long does a tooth have to be to accommodate an Occlusaly approaching clasp?

A

Greater than or equal 15mm to accommodate the 15mm clasp arm

29
Q

How long does a tooth a have to be to accommodate a gingivally approaching clasp?

A
  • Does not have to be greater than or equal to 15mm to accommo date the 15mm clasp arm
30
Q

What is the retentive arm of the clasp?

A
  • A flexible arm which deflects over the bulbosity to engage the undercut of the tooth
31
Q

What is the reciprocation component of the clasp?

A
  • This component counteracts the force of the retentive component with an equal and opposite force
32
Q

How much of the clasp can go below the survey line to engage the undercut?

A
  • Only the terminal 1/3 of the clasp
  • The other 2/3 seat above the survey line
  • Once the terminal 1/3 of the retentive arm has engaged the undercut it should be passive and no longer exert a force on the tooth
33
Q

Retention of the denture is dependent on how difficult it is to dislodge the clasps. The amount of force required is dependent on what 3 factors?

A
  • Flexibility of the retentive arm
  • Placement of the retentive arm
  • Depth of the undercut
34
Q

What is flexibility of the retentive arm of a clasp dependent on? (3 points)

A
  • Material (some materials are more flexible, others are more rigid)
  • Length (longer = more flexible)
  • Thickness (thinner = more flexible)
35
Q

What size of gauge do we use for a Co/Cr clasp when analysing undercuts?

A
  • 0.25mm
36
Q

What size of gauge do we use for a gold clasp when analysing undercuts?

A
  • 0.5mm
37
Q

What size of gauge do we use for a stainless steel clasp when analysing undercuts?

A
  • 0.75mm

BDS2 RPD (13 decks)

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